plants - PowerPoint by xiangpeng

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									Introduction to plants
All life on earth depends on plants. Without plants
ecosystems would soon grind to a halt.

Animals (and fungi) are parasitic on plants.
              Taxonomy - definitions
Always start by setting out what you‟re talking about!

Plants are widely taken to be the green things that make flowers

Kingdom Plantae – algae, mosses, ferns, conifers, flowering plants

In fact the boundaries are rather unclear, especially at the single-celled
level. Modern taxonomies do no try to shoehorn unicellular eukaryotes
into kingdoms alongside multicellular forms but prefer to handle them as
a distinct group (with 27 phyla at the last count). Within plants, the term
„alga‟ is not a monophyletic group, with red alga very different to brown
and green classes.
                         Basics
Plants (like animals and fungi) are eukaryotes. Plants are primitively
photosynthetic, relying on organelles called chloroplasts to capture
light energy. (A few plants have lost this ability and are parasitic on
other plants). Most have highly structured bodies, with green
material growing upwards and roots growing down. Their cell walls
are reinforced with tough polymers, notably cellulose and lignin.

All are capable of sexual reproduction, and their classification is
heavily based on studies of their reproductive organs. All exhibit a
phenomenon called alternation of generations, which you may well
be unaware of and which we will look at closely later in the lecture.
In this course
I will focus mainly on the familiar „higher‟ forms, flowering plants Algae
(and fungi) I will callously exclude, on grounds of time and factual
overload!

We will need a taxonomic structure to hang names on, so I propose to
start by introducing the major taxa you need to be familiar with. I am
sure that these divisions will remain valid – I am not sure that the exact
taxonomic levels will remain unchanged in the next 20 years. Geological
dates are also prone to being extended backwards as new fossils are
found.

“Each new edition of this book requires revision of most of the
phylogenetic trees to reflect the ongoing revolution in
systematics.”! Campbell Reece & Mitchell (1999) Biology (5th Edition)
Addison Wesley
(On p. 584 they show a family tree of photosynthetic eukaryotes which allows
for 4 different definitions of ‘plant’!)
    Classification of the plants we will cover in this course.


       Bryophytes    Pteridophytes     Gymnosperms:       Angiosperms:
       (mosses,      Ferns and         Conifers,          Flowering
       liverworts)   allies            cycads etc         plants



Mesozoic
120MYBP

Carboniferous
350MYBP
                                                 Seeds
Devonian
400MYBP                 Vascular tissues (tracheids or derivatives)
               Bryophytes
These are the mosses and liverworts, both relatively common well-
known groups of non-flowering plants typical of permanently damp
areas. (Actually a few specialise in dry open sites – fire sites, bare
concrete etc). The dominant phase is a leafy form (the gametophyte),
which is genetically different to the stalked pods that produce its spores.



                                                 A typical moss,
                                                 showing the spore
                                                 capsule, which is a
                                                 genetically different
                                                 plant to the green
                                                 fronds from which it
                                                 grows.
                                                 (More later..)
The hepatica: Liverworts. These also make genetically
distinct spore-dispersing individuals, but here the spores are
dispersed from an umbrella-like structure, while the main plant (the
gametophyte) is generally flattened, plate-like.


                                                      sporophytes

                            c. 2 cm




      The common liverwort
      Marchantia
        Pteridophytes: ferns, horsetails,
            club mosses and allies.
Pteridophytes are the group of plants which first (as far as we can tell…)
developed the tracheid cells which permit stems to rise high above any
water supply, and as such were the first colonists of dry land, at least 400
MYBP. We have a good fossil record of them (in fact our industry has
depended on burning this fossil record since the inception of the
industrial revolution!). The facets which fossilise show that apart from
the extinction of the giant forms, this group has changed little since the
Devonian.

Like mosses these plants have two genetically distinct phases in their life
cycle, but here the dominant phase is the sporophyte, the familiar fern
leaves etc.
                          Ferns
These are ancient but still successful forms, in which the spore-bearing
stage is very familiar. Bracken Pteridium aquilinum is one of the most
widespread and pernicious weeds on the planet! We still have tree ferns,
native to Gondwanaland (Australasia, South America, Africa) but now
widely planted in tropical, subtropical and frost-free temperate areas. In
all cases spores are shed from the underside of the leaves (fronds).

                    Bracken Pteridium aquilinum


                     A tree fern Dicksonia antarctica
Horsetails: Equisetacea
          These plants are every-day miracles. There
          are only about 15 species in the world, all
          in the genus Equisetum. It has changed
          hardly at all since the carboniferous period.
          I know of a Carboniferous site in Yorkshire
          where one can find 2m high horsetails still
          standing, fossilised in a cliff, looking
          exactly like living forms (only rather
          bigger, though giant horsetail E. telmateia
          can grow nearly this tall). Also known as
          Lego plants, because the stems comes apart
          at the nodes.
Horsetails, contd.   The needle-like leaves are
                     reinforced with silica, and have
                     been used as pan scrubs. Few
                     animals find them palatable.


                     For all their ancientness and oddity
                     they are a serious weed, with
                     immensely deep root systems and
                     an ability to shrug off herbicides.
                     Gardeners‟ Question Time (BBC)
                     advice on how to respond to
                     horsetails in your garden
                     Sell your house, in winter when
                     the stems aren’t visible.
  Club mosses:
  Lycopodiacea

These rather nondescript crawling
plants are nowadays confined to a
minor role in northern forests on
acid soils. Present in the UK but
easily overlooked. The sole
survivors of a large group including
vast forest-forming trees in the
carboniferous, the first terrestrial
forests. The have a vascular
system, and always one vein
running along the leaf axis.
                 Gymnosperms
This group contains many well-known plants, including all coniferous
trees (pines, larch, spruce etc), yews and allies, along with other „living
fossils‟ the cycads, plus a few simple plain oddities thrown in to keep
botanists happy.


Gymnosperm means „naked seed‟, and indeed in this group the
fertilised seed protrudes from the cone/aril. They have apparently lost
the sporophyte generation (but see later), and are now trees which shed
viable seed that germinates to make a new tree – the pattern of seed
germination which we are familiar with. They have tracheids allowing
water to be sucked to great heights: the this group contains probably
the largest (Sequoia) and oldest (Bristlecone pine, Pinus aristata)
organisms in the world.
                 UK Gymnosperms
We have 3 spp native to the UK: Scots pine Pinus sylvestris, Yew
Taxus baccata and Juniper Juniperus communis.

Do you know where they grow?

Scots pine – native to Scotland but widely naturalised on sandy acid
soils
Yew – dense ancient forests on southern chalk
Juniper – an oddly disjunct distribution, present but under Yew on
chalk and in the acid uplands.
                 Most people know one group of gymnosperms; the
 Conifers        conifers. Literally the cone bearers – these are
                 pines, spruces, larches, firs etc.
                     Cones – correctly stobili (1
                     strobilus) are sexual organs
                     either shedding pollen (male
                     cones) or bearing ovaries,
                     awaiting fertilisation by wind-
                     blown pollen (female cones).
                     In fact all gymnosperms, plus
Female pine cone     lycoods, have similar               male pine cone
                     structures. You will meet the
                     terms microsporangia (pollen-
                     producing organs) and
                     megasporangia (egg producing
                     organs).
Cycads   These plants look rather like stunted palms, or
         possibly rather tough tree ferns, but are neither. They
         are gymnosperms that have changed little since the
         Jurassic period, when they were dominant land cover
         and presumably staple food for herbivorous
         dinosaurs.

         Now they are thinly scattered in tropical areas, some
         highly endangered.

         Males and females plants are separate, using a wind-
         dispersed pollen to fertilise their cones. The male
         gamete is notable for using cilia to swim towards the
         egg (the „highest‟ occurrence of cilia in the plant
         kingdom).
         Some cycads fix atmospheric nitrogen using a
         symbiosis with blue-green algae living in their stems
         and roots.
Ginkgo biloba – the wonderful discovery
          People had since the early days of fossil hunting
          been recovering well-preserved fossil leaves from
          Ancient (Jurassic and earlier) which looked like an
          unrolled pine needle. No living plant matched this
          pattern.

          Then in 1691 the German Engelbert Kaempfer
          discovered strange trees with exactly this unfamiliar
          leaf form in Japan, cultivated in temple gardens.
          They proved to be living specimens of Ginkgo, one
          male and one female. Thankfully their seed was
          fertile, and has now been widely propagated. The
          oldest in the UK is in Oxford botanic gardens
          (pruned and now rather small for its age). Generally
          males are planted as the female flower is rather
          sticky and smelly. (Sex is coded by an X-Y
          chromosome system, as in mammals).
           Welwitschia mirabilis
This is certainly one of the strangest plants in the world, whose
classification inside the gymnosperms has long been assumed but is
confirmed by DNA analyses.

It lives only in the Namib desert, South Africa, in a region where rain
never falls. Instead it relies on the mist that condenses in coastal regions
where cold currents from the southern oceans well up against the desert.

Welwitschia has only 2 leaves, long strap-like ones that grow perpetually
from their base while the ends become frayed and tatty. It is dioecious.
     Angiosperms: Monocotyledons
           and Dicotyledons
Flowering plants (phylum Anthophyta) come in two fundamentally
different „designs‟ or classes, known as the Monocotyledons and
Dicotyledons. Or Monocots and Dicots in botanical jargon.

Formally these are defined by the number of seed leaves, or
cotyledons, that emerge when the seed 1st germinates. In
Monocotyledons it is 1, in Dicotyledons it is 2. Coinciding with
this are a series of other characteristics which are so consistent that
everyone seemed happy that these are monophyletic groups,
splitting from the gymnosperms about 130 MYBP (early
Cretaceous).
Unifying features of
monocots and dicots
   Class               Monocotyledon      Dicotyledon

   Number of cotyledons1                  2
   Leaf veination      parallel           branching
   Flower parts        in 3s              in 4s/5s
   Roots               a fibrous bundle   A tap root with laterals
   Stem                herbaceous         woody
   Arrangement of
   vascular bundles    scattered          in a ring
   Pollen pores        1                  3
               Monocotyledons

This truly monophyletic group contains all
grasses, sedges, rushes, bamboo etc. Orchids.
Pineapples and allies (the bromeliads). Lilies,
and their succulent relatives Aloes. Few trees
but including bananas and palms.
            Dicotyledons.       Actually the
           eudicotyledons plus a few others…
It is here that I have to confess to a certain oversimplification. Neat
though the division was, recent (late 1990s) DNA work has shown that
the group known as „Dicots‟ consists of 4 groups, all as unrelated to each
other as they are to the monocots. Fortunately, virtually all the ones you
are likely to meet are in a good monophyletic group, now called the
Eudicotyledons.

(Sometimes DNA research makes a good simple system needlessly
complicated..)
           Eudicotyledons

Here we have most gardens flowers, all
herbs, cacti, climbers, and most trees.
         The awkward other 3 groups
For sake of completeness I need to tell you of
the other 3 ex-dicots.

One group will be familiar and has long been
thought of as primitive: the water lilies
Nymphales. All are aquatic with floating leaves:
                                                   Nymphaea
                                                   Drimys winteri
A second early offshoot is the winterales,
containing Winters bark Drimys and Star Anise
Illicium.
                Amborella trichopoda
Until the Deep Green project looked at its DNA, Amborella was an
incredibly obscure and unimportant member of the laurel family only
found in the wild forests of New Caledonia.

It turned out to be closest to the root of all flowering plants, pre-dating
the split between monocots and the others. It has its own class now!
 Current angiosperm taxonomy.
 (Until they sequence something else really odd…)

                                                    Eudicots

                                                    Monocots

                                                    Winterales
                                                    Water lilies
Anthophyta:                                         Amborella
             Alternating generations:
          sporophytes and gametophytes

One of the more unusual insights into plant
biology comes from the study of their life
cycle, specifically in a pattern of alternation
that is easily seen among the „lower‟ forms          Gametophyte
but becomes progressively less obvious as            Sperm or eggs
one moves from ferns to cycads to conifers
and flowering plants.
                                                   0 0
This pattern consists of having two
genetically distinct generations, each giving
rise to the other, and is called the alternation
                                                         Sporophyte
of generations.
                                                         (sheds spores)
          Haploid N/diploid 2N?
There is one more feature to include here to understand a generalised
account of the alternation of plant generations.

This concerns the chromosomal composition: does the cell have 1 set of
chromosomes (haploid) or two (diploid)? Where in the cycle does the
crucial genetic shuffling act called meiosis occur?
                                  Gametophyte     Gametophyte
                                    Haploid N + N   Haploid

                              0 0
                         N

Meiosis to make haploid spores
                                              Sporophyte 2N
                                              (Diploid)
     This alternation is clearly seen in the non-vascular plants (the
     non-tracheophyta), mosses and liverworts.

     Here we see the dominant (at least the biggest, most visually
     conspicuous) part of the life cycle is the sporophyte, which
     shed spores from various shaped capsules. The taxonomy of
     these plants relies largely on this phase.

These spores germinate to produce a flat green individual, looking
very much like a smooth lichen. This is the gametophyte, and all
external gametophytes tend to look pretty much the same. There are
no field guides to gametophytes, as a fern is hard to tell from a
liverwort or a moss – until sporophytes appear.
This pattern can be seen right up through the plant kingdom. Various
modifications occur in the algae, such as the condition where the
sporophyte and gametophyte look identical (until you examine their
reproductive cells or count their chromosomes). This condition is
called isomorphic alternation of generations.

In mosses and liverworts the gametophyte thallus is the largest and
most metabolically significant phase. These are most easily seen in
liverworts, where the umbrella-like sporophytes sprout from an
expanse of flat green thallus. In these plants the spore is the main
agent of dispersal in time and space.

In these plants the male gametes are motile, swimming with cilia, and
can plausibly be called sperm.
In Pteridophytes the gametophyte generation (haploid) is still a distinct
individual (looking and behaving remarkably like a liverwort
gametophyte), but this is overshadowed, both literally and
metaphorically, by the much larger (diploid) sporophyte generation.
Fern collectors in the UK have collected-out some rare ferns, but their
gametophyte stage hung on overlooked and allowed the population to re-
emerge.

This pattern appears to halt with the arrival of seed plants –
gymnosperms and angiosperms.In fact we can see these as the extreme
development of the dominance of the 2N sporophyte generation. The
trick is to see the sporophyte as making a 1N gametophyte generation by
meiosis which is retained as an internal individual, an egg or a pollen
grain. The pollen is then dispersed, and on germination it releases a male
gamete which fertilises the egg. And the sperm? In cycads the gamete is
a ciliated ball which swims majestically towards the egg – elsewhere it is
replaced by a simple pollen tube along which the nucleus is propelled.
The alternation of generations –a summary




Taxon         Sporophyte 2N                 gametophyte N

Bryophyta     small capsules                dominant leafy phase

Pteridophyta Large leafy stage              small, liverwort-like

Seed plants   Only visible phase            tissue within reproductive
                                            organs

								
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